Winches are used extensively in hauling, pulling and hoisting machines for raising and lowering loads, particularly heavy loads. For example, they are commonly used in well drilling rigs to raise and lower drilling tools and/or sensing instruments. Winches include a drum that supports a spool of cable that runs to the load, usually through one or more sheaves. The cable is taken in and payed out by rotating the drum. Depending upon the environment in which the winch is to be used, the cable may be formed of steel or rope. In some instances, electrical signal conductors may form part of the cable.
As the cable is taken in it is wound (e.g., spooled) onto the drum in multilayers. Because cable is difficult to evenly spool, most winches include a spooling mechanism designed to control the fleet angle of the cable as cable is spooled onto the drum. In the past, most spooling mechanisms have been directly coupled to the winch drum such that the cable guide rollers of the spooling mechanism move in synchronism with the drum as the drum rotates. In the past this has caused a problem. Because cable varies in diameter and because cable diameter changes with use, very small timing errors occur between drum rotation and spooling mechanism movement. Because small timing errors accumulate, they become magnified, resulting in uneven spooling. Uneven spooling is undesirable because it causes more cable wear than even spooling. Uneven spooling is also undesirable because it is unsafe.
While proposals have been made to avoid uneven spooling, in general, they have not been entirely satisfactory, particularly in winches designed to raise the heavy loads associated with modern oil well rigs and ships. For example, U.S. Pat. No. 2,660,382, entitled "Levelwinding Device" by J. H. Wilson, describes a levelwind mechanism whose movement is dependent upon changes in fleet angle. Fleet angle changes are sensed and utilized to control the rotation of a sprocket that engages a chain to reduce fleet angle. One major disadvantage of this device is the fact that sprocket/chain mechanisms lack structural strength. Thus, while a device of the type described in U.S. Pat. No. 2,660,382 may be useful in connection with lightweight cable hoists, it is not suitable for use in mechanisms designed to hoist extremely heavy loads. Another disadvantage is the need for a carriage to move the levelwind mechanism perpendicular to the axis of rotation of the drum as the cable is taken in and payed out. In addition to making the overall device over complicated, the carriage has lower structural strength than a system that avoids the need for such a carriage.
This invention is directed to a levelwind mechanism that overcomes the foregoing and other problems of prior levelwind mechanisms, particularly levelwind mechanisms intended for use with hoists designed to raise heavy loads.